Devices such as a scanner and a printer output device values (like RGB values or CMYK values), which are values depending on the respective devices, or device-dependent values. To handle such device values, a color conversion table (a device profile) for converting device values into device-independent values is created, and color conversion (conversion between device values and device-independent values) is performed using the device profile. As an example of a way to create a device profile, a description is now given of a way to create a scanner profile. To create a scanner profile, a color chart is output with a printer, and the color chart is measured with a scanner and a color measurement device (like a colorimeter or a spectrophotometer), where the scanner obtains measured RGB values and the color measurement device obtains measured device-independent color values like L*a*b* values in the CIE 1976 color space or XYZ values of the CIE 1931 color space (“colorimetric values” is a general term for these values). The measured RGB values and the corresponding measured colorimetric values are associated with each other, whereby a scanner profile is created.
In executing color conversion of device values obtained by measurement of a printed matter with a scanner, by using a scanner profile created beforehand through the above-described procedures, a certain problem can arises. That is, an individual difference of a scanner used in the creation of the scanner profile and that used in color conversion (where the individual difference means a difference in characteristics among a same type of scanners which are different in lot) and a difference of print sheets in properties can cause deterioration in color accuracy of converted colors, even after color calibration has been executed in an assembly process of the printer, by using a reference color chart printed by offset printing. In such a case, it is necessary to correct the scanner profile.
As an example of a technique to correct a scanner profile, there is a technique to use a commercial software program, and recreate the scanner profile by measuring a color chart, which was reprinted with a printer, with both a scanner and a color measurement device. As an example of a technique to correct a scanner profile in response to an occurrence of deviation of scanner characteristics, Japanese Unexamined Patent Publication (JP-A) No. 2006-033572 discloses a technique to, when a need for color conversion arises because of deviation of scanner characteristics, cause a high-speed external device to perform calculation for a creation of a scanner profile, so as to reduce a load of an image processing apparatus.
As an example of a technique relating to correction of a device link profile, which is not a technique to correct a scanner profile, JP-A No. 2008-136065 discloses a technique to perform a color-tone correction on a device link profile which does not include device-independent profile data. The technique divides a device link profile into several parts according to K component values, to create three-dimensional parameters such that an output value is specified for each lattice point represented by input values for C, M, and Y components. From the three-dimensional parameters, the technique extracts at least one lattice point where any of C, M, and Y component values becomes 100%, as a target lattice point, and determines the range of full-saturated values (the range of tone-correction targets) in which output values are at full saturated. Then the technique adjusts the parameters at points needing adjustment, which are the lattice points in the determined range, so as not to give full-saturated tones.
To correct a scanner profile, the above-described technique using a commercial software program and the technique disclosed in JP-A No. 2006-033572 need a process to recreate a scanner profile from the beginning, and thus need sufficient time to correct the scanner profile. These techniques further need, at each time when a scanner profile is corrected, a process of outputting a color chart for creating a scanner profile, which may waste time and resources to print the color charts.
Moreover, the technique disclosed in JP-A No. 2008-136065 is a technique established under the assumption that input CMYK values are fixed. This technique is not suitable for correction of a scanner profile, because measurement of a color chart, which was printed by a printer having deviating characteristics, with a scanner and a color measurement device, gives deviating RGB values and deviating L*a*b* values.